[release-branch.go1.11] database/sql: correctly report MaxIdleClosed stat

Previously the MaxIdleClosed counter was incremented when added
to the free connection list, rather then when it wasn't added
to the free connection list. Flip this logic to correct.

Fixes #28325

Change-Id: I405302c14fb985369dab48fbe845e5651afc4ccf
Reviewed-on: https://go-review.googlesource.com/c/138578


Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 7db509e6)
Reviewed-on: https://go-review.googlesource.com/c/146697


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>